Sorry about that, I thought you were part of another thread about this. I have just published a lib that includes an easy to use CRC check that you can use to throw away bad results. Look at the example app and you will see what I mean.

The getTemp() function in DallasTemperature.cpp is what the getTempCByIndex() leads to. There you can see in the comments that if sensor.isConnected(DeviceID); is false OR !sensor.isConnected(DeviceID); it will return the DEVICE_DISCONNECTED value that is defined in DallasTemperature.h.